A honeycomb structure is often used as a filter, a catalyst carrier, or the like for, for example, an exhaust gas purification apparatus for a thermal engine such as an internal combustion engine or a combustion device such as a boiler, a reforming device for a liquid fuel or a gaseous fuel, a purification treatment device for service water and sewage, or the like. In particular, a honeycomb structure is suitably used as a diesel particulate filter (DPF) for trapping and removing particulate matter contained in dust-containing fluid such as exhaust gas discharged from a diesel engine or a high-temperature gas dust-trapping apparatus.
A honeycomb structure used for such a purpose traps and removes unnecessary particulate mater when fluid to be treated passes through fine pores in the porous partition walls or brings fluid to be treated into contact with a catalyst loaded on a porous partition wall surface or in the fine pores.
There has conventionally been conducted an inspection of clogging in cells (through-holes) serving as passages of fluid to be treated as one of methods of quality control of a honeycomb structure. A lighting apparatus for this inspection is disclosed (e.g., see Patent Document 1). This lighting apparatus for inspection is provided with a lighting means which has an irradiating angle, the first lens which collects light from the lighting means and converts the light into parallel light, and the second lens which collects the parallel light from the first lens after passing through the object to be inspected so that the light can be taken into an imaging means.
Patent Document 1: JP A 2003-130799.